Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “C.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 cdma2000 High Rate Packet Data Air Interface Specification” (the cdma2000 standard), and (4) some other standards.
In general, the performance of any CDMA system is enhanced as more fingers are added to receivers in order to process a greater number of multipath signals from one or many base stations. This is particularly true as the chip rate used to spread incoming signals increases, as more components of the multipath signal are then distinguishable at the receiver. Other methods to improve performance, such as receive diversity, in which multiple antennas are used to track received signals, require an increase in the number of fingers.
CDMA demodulators often include dedicated hardware, known as finger front ends, to process the relatively higher chip rate data that is received. Often a digital signal processor (DSP) or other processor is deployed to receive symbol rate data from the finger front end to further demodulate the symbols. One way to enhance the performance of any CDMA system, or to meet specifications for a higher chip rate system, is to replicate the hardware of one finger for as many fingers as are required. While this technique has been used with success in the past, as finger requirements grow, the resultant hardware requirements grow linearly in proportion to the number of fingers. Multiplying hardware for each finger can become prohibitively expensive.
In addition, a CDMA system may require certain minimum response times from a receiver to respond to a transmitted signal. This may impose a maximum latency allowed in demodulating received signals. One example is the forward power control bit punctured into the forward link data streams, as defined in the IS-95 and cdma2000 standards. Since CDMA systems are typically capacity constrained by interference generated by users within the system, to maximize capacity it is imperative that each mobile station responds to power control signals in a timely fashion to ensure that each user transmits only the necessary amount of power, thus minimizing interference.
There is therefore a need in the art for a CDMA demodulator that can be scaled to include additional fingers in a hardware efficient manner while maintaining any maximum demodulation latency requirements imposed by the system.